Technology is used heretofore that reads out an image by use of a sensor array that includes a plurality of sensor elements arranged in a row. In a magnetic reader for reading magnetic information printed with magnetic ink on a medium such as paper, a magnetoresistive (MR) sensor is used as a sensor element that converts magnetism to electrical information. The reading out of the image is enabled by using a switch to sequentially select among outputs of channel amplifiers, each of the channel amplifiers being connected to a respective sensor element.
The electrical signal output from the sensor element in such a reader has very low amplitude. Thus the channel amplifier that amplifies such an electrical signal is required to increase the signal level with high gain, and in order to secure a high signal-to-noise ratio (SNR), is required to suppress the level of noise generated by the amplifier. Further, technology concerned with the channel amplifier is known that obtains a high SNR by subtracting a DC component from the output signal of the sensor element (for example, Patent Literature 1).
Here, the bias current of the channel amplifier is high in order to realize low noise. Since the required number of channel amplifiers is the same as the number of sensor elements, for example, if the bias current of the channel amplifier is taken to be 1 mA and the power supply voltage is taken to be 5 V, then in a device using 500 sensor elements, the consumed current reaches 500 mA, and the consumed power reaches 2.5 W. Thus problems occur in that costs increase due to the high capacity of the power source supplying power to the device, and in that, due to heat generated by the device, working life of components installed within the device decreases.
To solve such problems, technology is proposed that lowers average power consumption by performing control that sets to the operating state only the channel amplifier for which the switch connected to the output of the channel amplifier is in the conductive state (closed), and at other times sets the channel amplifier to a low power consumption state (for example, Patent Literature 2). This lowering of power consumption is described as being realizable by setting the voltage of the bias source of an operational amplifier included in the channel amplifier to a low value.